Conventional hot dip galvanization consisting of dipping iron or steel articles in a molten zinc bath requires careful surface preparation, in order to assure adherence, continuity and uniformity of the zinc coating. A conventional method for preparing the surface of an iron or steel article to be galvanized is dry fluxing, wherein a film of flux is deposited on the surface of the article. Accordingly, the article generally undergoes a degreasing followed by rinsing, an acid cleaning also followed by rinsing, and a final dry fluxing, i.e. the article is dipped in a fluxing bath and subsequently dried. The basic products employed in conventional fluxing are generally zinc and ammonium chlorides.
It is well known that improvement in the properties of galvanized articles can be achieved by alloying zinc with aluminum. For example, addition of 5% aluminum produces a zinc aluminum alloy with the lowest melting temperature. This alloy exhibits improved fluidity properties relative to pure zinc. Moreover, galvanized coatings produced from this zinc-aluminum alloy have greater corrosion resistance (from two to six times better than that of pure zinc), improved formability and better paintability than those formed from pure zinc. Furthermore, galvanized coatings free from lead can be made with this technology.
However, the use of conventional fluxes in zinc-aluminum galvanizing leads to various defects in the coatings. In particular, some areas of the surface may not be covered, or not covered in a sufficient manner, or the coating may show outbursts, black spots or even craters, which give the article unacceptable finish and/or corrosion resistance. Thus, research has been carried out to develop fluxes that are more adapted to zinc-aluminum galvanizing. Despite these efforts, when it comes to the galvanizing of iron or steel articles in zinc-aluminum baths in batch operation, i.e. the galvanizing of individual articles, the known fluxes are still not satisfactory.